Adding Far-Red to Red, Blue Supplemental Light-Emitting Diode Interlighting Improved Sweet Pepper Yield but Attenuated Carotenoid Content

Research Article
Article Information

  • Title: Adding Far-Red to Red, Blue Supplemental Light-Emitting Diode Interlighting Improved Sweet Pepper Yield but Attenuated Carotenoid Content
  • Authors: Kim, D., & Son, J. E.
  • Published: 21 June 2022
  • Journal: Frontiers in Plant Science
    • Volume: 13 
    • Issue: 938199
  • DOI: https://doi.org/10.3389/fpls.2022.938199

Research Background


A. Research Context


  • Sweet peppers (Capsicum annuum L.) are globally valued for their nutritional content, including vitamin C and carotenoids, which influence fruit quality and human health benefits (e.g., antioxidant properties, vitamin A conversion).
  • In greenhouses, light is a limiting factor for year-round production, especially in northern latitudes with low winter sunlight. Supplemental LED interlighting (red + blue spectra) is commonly used to enhance photosynthesis and yield.
  • Recent studies suggest far-red (FR) light (700–750 nm) may improve fruit yield (e.g., in tomatoes) by influencing phytochrome-mediated responses (e.g., stem elongation, sugar partitioning), but its effects on sweet pepper fruit quality (e.g., carotenoids, sugars) remain unclear.

B. Knowledge Gap


  • While red/blue LED interlighting boosts sweet pepper yield, the impact of adding FR light on carotenoid accumulation and nutrient quality is poorly understood.
  • Prior research focused on tomatoes or controlled environments, leaving a gap in understanding spectral interactions (red/blue/FR) under real greenhouse conditions with natural light fluctuations.

C. Research Objectives


  • Compare the effects of three light treatments on sweet peppers:
    • Natural light (NL) (control).
    • NL + red/blue LED interlighting (RB).
    • RB + far-red LED interlighting (RBFR).
  • Evaluate impacts on fruit yield, physicochemical quality (sugars, ascorbic acid), and carotenoid content in red/yellow cultivars.

D. Significance


  • Addresses the trade-off between yield and nutritional quality under supplemental lighting.
  • Guides growers in spectrum selection (e.g., prioritize yield with FR vs. carotenoids with RB).

E. Additional Context


  • Phytochromes (photoreceptors) mediate FR responses:
    • FR shifts phytochrome equilibrium (Pfr → Pr), potentially suppressing carotenoid genes (PSY1) via PIF transcription factors.
  • Carotenoids (e.g., capsanthin in red peppers, lutein in yellow peppers) are light-sensitive pigments critical for color and health benefits.

Materials and Methods


1. Plant Materials & Growth Conditions


  • Cultivars: Red (Mavera) and yellow (Florate) sweet peppers (Capsicum annuum L.), grown in a Venlo-type greenhouse (Seoul National University, South Korea).
  • Timeline:
    • Sowing: Seeds planted in stonewool plugs, germinated under controlled nursery conditions.
    • Transplanting: Moved to stonewool slabs (6-week-old seedlings, August 2020).
    • Light Treatments: Applied from 50 days after transplanting (DAT) until harvest.
  • Environmental Controls:
    • Daytime temp: 25°C, humidity: 56%.
    • Nutrient solution (EC: 3.0 dS m⁻¹, pH: 5.8).

2. Light Treatments


  • Three treatments tested (12h/day, 6:00–18:00):
    • NL: Natural light only (control).
    • RB: NL + red/blue LED interlighting (71 µmol m⁻² s⁻¹, R:B = 8:2).
    • RBFR: RB + far-red LED (55 µmol m⁻² s⁻¹).
  • Setup: LED fixtures installed at 90 cm and 110 cm above beds.

3. Data Collection


  • Fruit Sampling:
    • Group 1: Harvested Nov 2020 (high natural light period).
    • Group 2: Harvested Jan 2021 (low natural light period).
    • Measured yield, individual fruit weight, size (length/width).
  • Light Measurements:
    • PPFD (photosynthetic photon flux density) and spectral distribution (400–750 nm) recorded at canopy levels (upper/middle/lower).

4. Fruit Quality Analysis


  • Physicochemical Traits:
    • Total soluble sugars (TSS): Refractometer.
    • Ascorbic acid: HPLC with UV detection.
    • Firmness: Texture analyzer.
  • Carotenoids:
    • Quantified 9 carotenoids (e.g., capsanthin, lutein) via HPLC.

5. Statistical Analysis


  • Software: R (v3.6.1) with Agricolae package.
  • Tests: One-way ANOVA, Tukey’s HSD (P < 0.05).

Key Notes on Methodology


  • Controlled Variables: Light spectra were calibrated to ensure consistent R:FR ratios (RBFR = 0.91 vs. RB = no FR).
  • Real-World Relevance: Experiments mimicked commercial greenhouse conditions with dynamic natural light.
  • Precision: HPLC and spectroradiometry ensured high-accuracy nutrient and light data.

Results & Discussions


1. Fruit Yield & Morphology


  • RBFR Increased Yield:
    • Red peppers: 33% higher yield vs. NL, 9% higher vs. RB.
    • Yellow peppers: 21% higher yield vs. NL, 19% higher vs. RB.
  • Fruit Size: RBFR maintained larger fruit (length/width) compared to RB, which reduced individual fruit weight.

2. Fruit Quality


  • Soluble Sugars (TSS):
    • RB and RBFR increased TSS (up to 25% higher in yellow peppers vs. NL).
    • RBFR showed lower TSS than RB in Group 2, suggesting FR may dilute sugar concentration despite higher yield.
  • Ascorbic Acid:
    • Consistently higher under RBFR (e.g., 9% increase in red peppers vs. RB).

3. Carotenoid Content


  • RB Boosted Carotenoids:
    • Red peppers: 3.0-fold higher vs. NL.
    • Yellow peppers: 2.1-fold higher vs. NL.
  • RBFR Reduced Carotenoids:
    • 26% lower in red peppers vs. RB; 9% lower in yellow peppers.

4. Light Spectrum Effects


  • RBFR Enhanced Canopy Penetration: Far-red enriched light reached lower canopy, improving yield.
  • Ripening Speed: RBFR accelerated coloration in Group 2 (5–10 days earlier).

5. Discussion Points


  • Trade-Off: Yield vs. Nutritional Quality
    • FR Light Prioritizes Yield: RBFR’s higher yield aligns with phytochrome-mediated growth responses (e.g., stem elongation, sugar partitioning).
    • Carotenoid Suppression: FR shifts phytochrome equilibrium (Pfr → Pr), potentially downregulating carotenoid genes (PSY1) via PIF transcription factors.
  • Light Quantity vs. Quality
    • Sugars & Ascorbic Acid: More responsive to total light intensity (PAR), explaining similar RB/RBFR results.
    • Carotenoids: Sensitive to spectral ratios (R:FR), with red/blue light being critical for biosynthesis.
  • Practical Implications
    • For High Yield: Use RBFR (e.g., winter production).
    • For Nutrient Density: Use RB alone (maximizes carotenoids).
    • Dynamic Lighting: Adjust spectra seasonally (e.g., RBFR in low-light periods, RB during fruit maturation).
  • Limitations & Future Work
    • Natural Light Variability: Seasonal changes affected results (Group 1 vs. Group 2).
    • Chloroplast Dynamics: Unmeasured factors (e.g., chloroplast movement under FR) may influence outcomes.

Conclusion


1. Far-Red Light Enhances Yield


Adding far-red (FR) to red/blue (RB) interlighting (RBFR) increased sweet pepper yields by 9–33% compared to RB alone, demonstrating FR’s role in improving productivity under low-light conditions.

2. Trade-Off with Carotenoids


While RBFR boosted yield, it reduced carotenoid content by 9–26% compared to RB, highlighting a critical trade-off between quantity and nutritional quality.

3. RB Alone Maximizes Nutrient Density


Red/blue interlighting (RB) without FR significantly elevated carotenoids (up to 3-fold higher than natural light), making it ideal for nutrient-focused production.

4. Light Spectrum Tailoring Required


The optimal spectrum depends on cultivation goals:
  • Yield-driven: Use RBFR.
  • Nutrition-driven: Use RB.
  • Seasonal adaptation: Combine strategies (e.g., RBFR in winter, RB in summer).

5. Final Statement


This study demonstrates that far-red light is a double-edged sword in sweet pepper production—enhancing yield but compromising carotenoid accumulation. Growers must strategically balance light spectra to align with market demands, prioritizing either productivity or nutritional value. Future research should explore dynamic lighting systems to optimize both yield and quality in real-time.